EP1760065B1 - Process for isolating vinyl acetate - Google Patents

Abstract

The separation of vinyl acetate from a gas mixture, comprises feeding the mixture left from a reaction zone into a first distillation column, cooling down (-20 to + 50[deg]C) the mixture discharged at the column head to form a first condensate separated into water phase (42) and first organic phase (44), and removing the water phase. The gas mixture from the zone is cooled down in a counter-current heat exchanger (9) with a colder cycle gas to 115-150[deg]C. The first organic phase as feedback is totally/partly fed back to the column head. The separation of vinyl acetate from a gas mixture, comprises feeding the mixture left from a reaction zone into a first distillation column, cooling down (-20 to + 50[deg]C) the mixture discharged at the column head to form a first condensate separated into water phase (42) and first organic phase (44), and removing the water phase. The gas mixture from the zone is cooled down in a counter-current heat exchanger (9) with a colder cycle gas to 115-150[deg]C. The first organic phase as feedback is totally or partly fed back to the column head, and a part of the organic phase not used as feedback is removed. The gas mixture is formed in a gas phase during a reaction of ethylene with acetic acid and oxygen at a catalyst containing palladium or palladium compounds. A gas containing non-condensed vinyl acetate is washed in a washing column with aqueous acetic acid (90%) to obtain the acid containing the vinyl acetate at a sump. The sump product containing vinyl acetate, ethyl acetate, acetic acid and water, is supplied to a collecting tank (14), and a liquid is primarily relaxed at 0.1-0.15 MPa to from a gas. The relaxed liquid is supplied to a second distillation column to remove a side stream containing ethyl acetate from an enrichment zone above the sump of the column. The removed side stream is totally or partly used for the gas washing in the washing column. A head vapor removed from the side stream is cooled down to form second condensate separated into water and second organic phases, and the water phase is removed. A part of the second organic phase is fed back to the head of the second distillation column, and the non-fed back second organic phase is removed. A part of the acetic acid solution containing the vinyl acetate is cooled through a pumping (15) in a closed cycle and the cooled solution is fed back to the sump of the washing column. The uncooled solution is heated at 60-100[deg]C and the heated solution is delivered to 5 t>h to 10 t>h column plates of the first distillation column bottom. The sump discharge of the first distillation column exhibits a temperature of 80-150[deg]C. The first non-fed back organic phase is secondarily relaxed at 0.1- 0.15 MPa in relaxation container (56) to from a gas combined with the gas from the primary relaxation. The head vapor of the second distillation contains acetic acid and ethyl acetate. The secondarily relaxed organic phase is combined with the second fed-back organic phase in a phase separator containing ethyl acetate. A part of the non-fed back second organic phase removed is directed into a third distillation column (48). A head product of the third distillation column is cooled down to form low boiling fraction and water. A sump product of the third distillation column is directed into a fourth distillation column to remove the pure vinyl acetate at the fourth distillation column head.

Description

The present invention relates to a process for the isolation of vinyl acetate from the gas mixture, which is formed in the reaction of ethylene with acetic acid and oxygen from palladium or palladium compounds containing catalysts in the gas phase, with recycling of the obtained in the recycle gas scrubber acetic acid solution in the first distillation column.

The preparation of vinyl acetate by reacting ethylene with acetic acid and oxygen or oxygen-containing gases in fixed bed catalysts in the gas phase is already known. The reaction is generally carried out at pressures of 1 to 2.5 MPa and temperatures of 100 to 250 ° C. Suitable catalysts contain a precious metal portion and an activator portion. The noble metal content consists of palladium and / or its compounds; In addition, gold or its compounds can still be present. The Aktivatoranteil consists of compounds of elements of the 1st main group and / or the 2nd main group and / or cadmium. These active components are applied to carriers in a fine distribution, wherein as the carrier material is generally used silica or alumina.

In general, the palladium content in the catalyst is between 0.5 and 5 wt .-%.

If gold or one of its compounds is used, it is added in a proportion of 0.01 to 4% by weight.

Each individual activator is also generally added in a proportion of 0.01 to 4% by weight. For all three percentages, the metal content of the component is in each case based on the total mass of the supported catalyst. The following catalysts are preferred: palladium / alkali metal element / cadmium and palladium / gold / alkali metal element, it being possible for palladium or gold to be present as metals or compounds in the finished catalyst and potassium being preferred as the alkali metal element. Potassium is used in the form of a carboxylate, especially as acetate.

The catalysts are particularly preferably palladium acetate / potassium acetate / cadmium acetate, as well as palladium acetate / barium acetoaurate / potassium acetate.

In the multistage catalytic process, vinyl acetate and water are produced in equimolar amounts, as shown in the following equation:

The unavoidable total oxidation of ethylene produces CO ₂ and water:

H ₂ C = CH ₂ + 3 O ₂ → 2 CO ₂ + 2 H ₂ O

It therefore accumulates more than 1 mole of water per mole of vinyl acetate; In general, the weight of the water makes up about a quarter of the weight of the vinyl acetate formed.

Apart from CO ₂ , small amounts of other by-products, including ethyl acetate, are formed in an amount of about 1000-2000 ppm by weight, based on the vinyl acetate formed.

Ethyl acetate may be contained in pure vinyl acetate only in a minor proportion of at most 250 ppm by weight. The separation of ethyl acetate requires a great deal of energy and the prior art treats various methods of reducing the energy expenditure in the purification of vinyl acetate to remove ethyl acetate and other by-products.

The mixture used for the reaction contains a multiple of the stoichiometrically required amount of ethylene. Accordingly, the ethylene conversion is relatively low at about 10% and the unreacted ethylene must be recycled to the reaction zone. Vinyl acetate is usually separated in a multi-stage process from the mixture of gaseous reaction products.

In the method according to DE-A1-3 422 575 the hot gas mixture leaving the vinyl acetate reactor and consisting essentially of ethylene, acetic acid, vinyl acetate, water, carbon dioxide, oxygen and inerts such as nitrogen and argon containing the ethyl acetate is introduced into a first distillation column operating without additional heating, in which so-called pre-dewatering column. The gas mixture emerging at the top of this column is first brought into contact with the return to the pre-dewatering column in a heat exchanger, with the gas mixture cooling and the return being heated accordingly. The gas mixture then passes from the heat exchanger into a condenser. The liquefied components are collected in a collecting tank, in which it comes to the separation into the aqueous and organic phase. The aqueous phase is discharged while the organic Phase is completely or partially recycled as reflux to the head of the pre-dewatering column.

The non-liquefied in the condenser shares still contain gaseous vinyl acetate. This is washed out of the gas mixture in a scrubbing column operated with acetic acid as a scrubbing liquid, which is also referred to as a recycle gas scrubber. The remaining residual gas is returned to the reactor. The bottoms of the recycle gas scrubber and the rest of the liquefied from the condensate of the pre-dewatering column organic phase is collected in another container, unless the entire condensed from the condensate organic phase is used as reflux in the pre-dewatering column.

At the bottom of the pre-dewatering column, a mixture is obtained which consists of vinyl acetate, acetic acid and about half of the water of reaction and by-products. The other half of the water of reaction has already been separated without energy supply and forms the aqueous phase of the condensate, which is formed during the cooling of the overhead vapor of the pre-dewatering column.

The bottom product of the pre-dewatering column is first fed to a collecting tank, which is also referred to as Rohvinylacetatsammelbehälter, and then worked up in a second distillation column, the so-called azeotrope. It precipitates water-saturated vinyl acetate as the top product, an ethyl acetate-containing side stream and a bottom product, which is recycled as the back acetic acid in the system. The ethyl-containing side stream is discharged. The water-saturated vinyl acetate, which is not added as reflux to the top of the second distillation column, is combined with the bottoms effluent of the recycle gas scrubber and the remainder of the organic phase liquefied from the condensate of the pre-dewatering column.

Subsequently, the mixture is added to a further third distillation column, to the so-called dewatering column. The overhead vapor on this column is almost completely returned as reflux after condensation. The side stream is separated into an aqueous and organic phase, in which case the aqueous phase is discharged and the organic phase is added to the column again. At the bottom of the dewatering column, a dry vinyl acetate / acetic acid mixture is withdrawn and added to a further fourth column, to the so-called Reinvinylacetatkolonne. In this column falls as the top product almost ethyl acetate-free vinyl acetate, while the bottom of this column, which contains acetic acid, high boilers and traces of vinyl acetate and ethyl acetate, is recycled under discharge of a partial flow back into the process.

Another variant of the known process for working up vinyl acetate is made EP-A2-0 423 658 known. Thereafter, the bottom product of the cycle gas scrubber is not combined directly with the obtained at the azeotrope aqueous vinyl acetate but first introduced into a further column in which the head product is a vinyl acetate-water azeotrope and acetic acid as the bottom product, which is recycled to the process. The obtained in this additional column aqueous vinyl acetate is combined with the obtained from the azeotrope water-saturated vinyl acetate and according to the method according to DE-A1-3 422 575 worked up in the downstream dewatering column and pure vinyl acetate. The method according to EP-A2-0 423 658 requires about the same amount of distillation energy for ethyl acetate separation as the method according to DE-A1-3 422 575 However, a smaller number of column bottoms is required, which means less investment costs. The uncondensed part of the vinyl acetate from the pre-dewatering column, which is washed out in the cycle scrubber with acetic acid and obtained as an acetic acid solution, and the organic phase of the condensate from the pre-dewatering column contain almost no ethyl acetate more and an energetically complex liberation of vinyl acetate streams of ethyl acetate is eliminated. However, this process variant requires the operation of an additional distillation column for separating the bottoms effluent from the recycle gas scrubber.

The known work-up procedures for the recovery of pure vinyl acetate still have certain disadvantages. Thus, the bottom outlet of the cycle gas scrubber and the bottom outlet of the pre-dewatering column contains considerable amounts of gases dissolved, especially of ethylene. In the relaxation of the bottoms effluent from the pre-dewatering column and from the recycle gas scrubber in Rohvinylacetatsammelbehälter therefore a considerable amount of return gas is released, which is aufkomprimieren with high energy expenditure in a recycle gas compressor before it can be recycled back into the reaction cycle. Generally, the crude vinyl acetate is depressurized from a pressure in the range of 0.5 to 2.0 MPa to a pressure of 0.02 to 0.2 MPa. The gas formed during the expansion contains predominantly ethylene as well as carbon dioxide, nitrogen and other inert substances such as argon and organic constituents such as acetic acid and small amounts of vinyl acetate and ethyl acetate. This gas is also referred to as the return gas, which is returned to the process.

Characteristic of the known work-up process is the merging of the acetic acid solution resulting from the bottoms of the recycle gas scrubber with the water-saturated vinyl acetate from the top product of the Azetropkolonne and the rest of the liquefied from the condensate of the pre-dewatering column organic phase. Therefore, the further purification stages, which take place in the downstream dehydration column and pure vinyl acetate, an acetic acid mixture is supplied, must be separated from the under high energy expenditure of acetic acid. In addition, drainage column and pure vinyl acetate column must be designed with corrosion resistant materials that are insensitive to acetic acid.

Also contains the condensate of the pre-dewatering column, which is not returned as reflux to the head of the pre-dewatering column, nor a certain amount of ethyl acetate. Since this stream is brought together only after the azeotrope with the top product obtained there, water-saturated vinyl acetate, the downstream dewatering and Reinvinylacetatkolonne an ethylacetathaltiger material stream is supplied from the ethyl acetate can be separated only with high energy expenditure.

Finally, an improved separation of water and ethyl acetate at a possible upstream processing step is desirable in order to prevent the carry-over of these undesirable substances throughout the work-up process as possible and to avoid the associated, energy-consuming separation in the pure vinyl acetate.

1. a) das aus der Reaktionszone austretende Gasgemisch in eine erste Destillationskolonne einleitet,
2. b) das am Kopf der ersten Destillationskolonne austretende Gasgemisch auf -20 bis +50°C abkühlt, wobei sich das anfallende Kondensat in eine Wasserphase und in eine organische Phase trennt,
3. c) die in Schritt b) gebildete Wasserphase abzieht,
4. d) die in Schritt b) gebildete organische Phase ganz oder teilweise als Rücklauf auf den Kopf der in Schritt a) benutzten ersten Destillationskolonne zurückführt und einen nicht als Rücklauf verwendeten Teil der organischen Phase abzieht,
5. e) das in Schritt b) nicht kondensierte, Vinylacetat haltige Gas in einer Waschkolonne mit mindestens 90%iger wässriger Essigsäure wäscht und dabei am Sumpf eine Vinylacetat haltige, essigsaure Lösung gewinnt,
6. f) das Vinylacetat, Ethylacetat, Essigsäure und Wasser enthaltende Sumpfprodukt von Schritt a) einem Sammelbehälter zuführt und die unter Druck stehende Flüssigkeit entspannt, wobei sich ein Gas bildet,
7. g) die bei der Entspannung in Schritt f) anfallende Flüssigkeit einer zweiten Destillationskolonne zuführt und aus einer Anreicherungszone oberhalb von deren Sumpf einen Ethylacetat haltigen Seitenstrom abzieht,
8. h) das Essigsäure und Wasser enthaltende Sumpfprodukt von Schritt g) ganz oder teilweise zur Gaswäsche in Schritt e) benutzt,
9. i) den Kopfdampf von Schritt g) abkühlt, wobei sich das anfallende Kondensat in eine wässrige und eine organische Phase trennt,
10. j) die in Schritt i) gebildete wässrige Phase abzieht,
11. k) einen Teil der in Schritt i) gebildeten organischen Phase als Rücklauf auf den Kopf der in Schritt g) benutzten zweiten Destillationskolonne zurückführt und den restlichen Teil abzieht, dadurch gekennzeichnet, dass man
12. l) einen Teil des Sumpfproduktes der in Schritt e) benutzten Waschkolonne unter Umpump zunächst kühlt und in den Sumpf der in Schritt e) benutzten Waschkolonne zurückführt, den restlichen Teil abzieht und den abgezogenen Teil auf eine Temperatur von wenigstens 30°C erwärmt und auf den unteren Teil der in Schritt a) benutzten ersten Destillationskolonne gibt,
13. m) die in Schritt d) abgezogene restliche organische Phase entspannt, das bei der Entspannung gebildete Gas mit dem in Schritt f) gebildeten Gas vereinigt und das gemeinsame Gas wieder in den Prozess zurückführt,
14. n) die in Schritt m) anfallende organische Phase mit der in Schritt i) anfallenden organischen Phase vereinigt und den in Schritt k) restlichen abgezogenen, nicht als Rücklauf verwendeten Teil der organischen Phase in eine dritte Destillationskolonne einleitet,
15. o) das Kopfprodukt der dritten Destillationskolonne aus Schritt n) abkühlt und die dabei anfallenden Leichtsieder und das dabei anfallende Wasser abtrennt,
16. p) das Sumpfprodukt der dritten Destillationskolonne aus Schritt n) in eine vierte Destillationskolonne einleitet,
17. q) am Kopf der in Schritt p) verwendeten vierten Destillationskolonne reines Vinylacetat abzieht.

The invention therefore consists in a process for the separation of vinyl acetate from the gas mixture, which is formed in the reaction of ethylene with acetic acid and oxygen on catalysts containing palladium or palladium in the gas phase, wherein

1. a) initiating the gas mixture leaving the reaction zone into a first distillation column,
2. b) the gas mixture leaving the top of the first distillation column is cooled to -20 to + 50 ° C., the resulting condensate separating into a water phase and into an organic phase,
3. c) withdrawing the water phase formed in step b),
4. d) the organic phase formed in step b) is wholly or partly recycled as reflux to the top of the first distillation column used in step a) and a non-recycled part of the organic phase is withdrawn,
5. e) washing the uncondensed in step b), vinyl acetate-containing gas in a wash column with at least 90% aqueous acetic acid and thereby wins a vinyl acetate containing acetic acid solution at the bottom,
6. f) the bottoms product containing vinyl acetate, ethyl acetate, acetic acid and water from step a) is fed to a collecting container and the pressurized liquid is released, forming a gas,
7. g) supplying the liquid obtained during the expansion in step f) to a second distillation column and withdrawing an ethyl acetate-containing side stream from an enrichment zone above the bottom thereof,
8. h) using the acetic acid and water-containing bottom product from step g) wholly or partly for gas scrubbing in step e),
9. i) cooling the overhead vapor from step g), the resulting condensate separating into an aqueous and an organic phase,
10. j) withdrawing the aqueous phase formed in step i),
11. k) recycling a portion of the organic phase formed in step i) as reflux to the top of the second distillation column used in step g) and withdrawing the remaining portion, characterized in that
12. l) a part of the bottom product of the wash column used in step e) initially cools and circulates in the bottom of the wash column used in step e), the remaining part is withdrawn and the withdrawn part to a temperature of at least 30 ° C and heated to the lower part of the first distillation column used in step a),
13. m) the residual organic phase withdrawn in step d) is expanded, the gas formed during the expansion is combined with the gas formed in step f) and the common gas is returned to the process,
14. n) the organic phase obtained in step m) is combined with the organic phase obtained in step i) and the fraction of the organic phase not withdrawn in step k) is introduced into a third distillation column,
15. o) the top product of the third distillation column from step n) is cooled and the resulting low boilers and the resulting water are separated off,
16. p) initiating the bottom product of the third distillation column from step n) into a fourth distillation column,
17. q) pure vinyl acetate is stripped off at the top of the fourth distillation column used in step p).

In step a), the gas mixture leaving the reaction zone is preferably first cooled to 115 ° C.-150 ° C. in countercurrent heat exchange with the colder circulating gas which is heated therewith and then returned to the reaction. In this case, no condensation of the liquefiable fractions occurs and the gas mixture is introduced into the first distillation column, also known as a predewatering column.

The amount of the organic phase formed in step b) is dependent on which temperature is cooled in this step. The part of the organic phase from step b) which is not used as reflux for step d) is withdrawn and depressurized in step m) from a pressure of 0.5 to 2.0 MPa to a pressure of 0.02 to 0.2 MPa, preferably 0.1 to 0.15 MPa. The resulting liquid is combined in step n) with the organic phase from the condensed overhead product of the second distillation column, also called azeotropic column (step i). Preferably, both organic phases are combined in the phase separator of the azeotropic column. The fraction of the organic phase which is not added as reflux to the top of the azeotrope column in step n) is introduced into a third distillation column, which is also referred to as a dehydration column.

The cooling temperature in step b) and the proportion of the organic phase formed in b) are preferably chosen such that as little vinyl acetate as possible, but preferably all of the ethyl acetate, are contained in the bottom product of step a).

Characteristic of the procedure according to the invention is the operation of the scrubbing column used in step e) and the recirculation of the bottoms effluent from the scrubbing column to the lower part of the first distillation column used in step a). A part of the bottom effluent from the scrubbing column, which is also referred to as a recycle gas scrubber, is circulated by pumping, whereby the part of the bottom product guided in the recirculation is cooled from the scrubbing column. To cool the bottom product, agents are used which are customary to the person skilled in the art, for example heat exchangers. The non-recirculated part of the bottoms product is withdrawn from the wash column, heated to a temperature of at least 30 ° C, preferably from 60 ° C to 120 ° C, especially from 60 ° C to 100 ° C, and to the lower part the first distillation column used in step a) driven. Conveniently, the bottoms product pumped out of the wash column is passed through a heat exchanger for heating.

Preferably, the heated bottom effluent from step I) is added to the 2nd to 15th, in particular to the 5th to 10th floor, calculated from the bottom of the column, the first distillation column.

By returning the heated bottom product from the scrubbing column from step e) to the lower part of the first distillation column used in step a), the temperature of the bottom effluent of the scrubbing column, the temperature of which is generally 30 to 50 ° C without this measure, significantly increased , In a first step, the sump outlet, for example in a heat exchanger, is first heated to a temperature of at least 30 ° C, preferably from 60 ° C to 120 ° C and especially from 60 ° C to 100 ° C, heated. In the task of the thus heated bottoms of the cycle gas scrubber on the lower part of the first distillation column of this stream is heated again, generally to a temperature of 80 ° C to 150 ° C, which also corresponds to the temperature of the bottom effluent of the first distillation column. This heating of the bottom effluent of the wash column reduces the solubility of the gaseous components in the acetic acid, crude vinyl acetate. The gaseous components, in particular ethylene and carbon dioxide, are increasingly expelled via the top of the first distillation column and returned to the gas circulation at a very early point in the work-up process. In the relaxation of the crude product therefore less gas is generated. The relaxation takes place in a collection, also called Rohvinylacetatsammelbehälter, from a pressure of 0.5 to 2.0 MPa to a pressure of 0.02 to 0.2 MPa, preferably 0.1 to 0.15 MPa. The resulting in the expansion gas is also referred to as the return gas and it contains predominantly ethylene and carbon dioxide and other inerts such as nitrogen and argon, as well as organic components such as acetic acid and small amounts of vinyl acetate and ethyl acetate. In the return of the return gas in the process, therefore, a lower energy consumption is required to compress the return gas back to the pressure stage of the reactor. The recycling of the bottoms effluent from the wash column from step e) according to the invention therefore relieves the recycle gas compressor, which brings about a significant energy saving.

Furthermore, by the task of acetic acid solution from the wash column to the lower part of the first distillation column, preferably on the 2nd to 15th floor, in particular on the 5th to 10th floor, calculated from the bottom of the column, achieved a washing effect. Ethyl acetate is washed in the bottom of the first distillation column and discharged via the bottom.

Vinyl acetate is present in the bottom product of the first distillation column, in the acetic acid wash solution formed in step e), which is recycled in step 1) to the lower part of the first distillation column and in the portion not used as reflux in step d) in step b ) formed organic phase. The vinyl acetate content in these three streams depends on the plant procedure and is not critical to the implementation of the method according to the invention.

The top product of the first distillation column contains only very small amounts of ethyl acetate and the recycled in step d) and the non-recycled part of the organic phase is low in ethyl acetate and can be further processed without further measures requiring an ethyl acetate separation. For this purpose, the stripped organic phase is relaxed after step m) and the resulting liquid combined with the obtained in step i) organic phase, which is from the top product of the second distillation column, also called azeotropic column, is obtained. The combined organic phases are partly recycled as reflux to the top of the azeotrope column. The remainder is added to the third distillation column, also called a dewatering column (step n).

The resulting in the relaxation in step m) gas is also referred to as a return gas and has about the same composition as the in step f) accumulating return gas. Both return gas streams are combined, then compressed in a recycle gas compressor and then returned to the process. Expediently, the combined return gas is combined with the residual gas obtained in the acetic acid wash in step e), which is also referred to as recycle gas. The combined gas streams are compressed and recirculated to the vinyl acetate reactor after removal of an inert-containing fraction.

In the gas scrubbing of step e), at least some of the bottom product of the second distillation column (step g) is used. The bottom product consists mainly of acetic acid and contains at most 10% by weight of water. A portion of the bottoms product not required in step e) is preferably recycled to the reactor as back acetic acid after a small portion has been removed to remove high boilers and polymers.

In step n), it is preferable to recycle only as much of the organic phase recombined in step i) and m) that the top vapor of the second distillation column contains as little ethyl acetate as possible. That part of the organic phase which is not required for this purpose is introduced into the third distillation column, also called a dehydration column.

In step o), the condensed overhead product of the third distillation column is not used completely as reflux, but a portion sufficient for low boiler and water removal is withdrawn.

In step p), the bottom effluent of the third distillation column, which consists almost of dry vinyl acetate, is added to a fourth distillation column, the so-called pure vinyl acetate column, on which pure vinyl acetate is taken off as top product (step q).

The first, second, third and fourth distillation columns used in the claimed work-up procedure of vinyl acetate are operated at temperature, pressure and reflux ratios resulting from plant utilization.

The inventive method is by FIG. 1 explained. Known measures, such as stabilizer additive, are not shown.

The composed of ethylene, oxygen and CO ₂ and inert and low organic content, such as acetic acid, existing recycle gas mixture, also referred to as recycle gas is passed via the line (1) in a formed as a plate column acetic acid evaporator (2), in which the gas stream with Acetic acid is charged, which is supplied via line (3). The gas mixture leaving the acetic acid evaporator (2) is fed to the vinyl acetate reactor (5) via a steam-heated line (4).

The gaseous mixture leaving the vinyl acetate reactor (5), consisting essentially of ethylene, acetic acid, vinyl acetate, water, carbon dioxide, oxygen and inert gases, such as nitrogen and argon, is passed via line (6) to the first distillation column, the pre-dewatering column (7). The pre-dewatering column (7) is designed in a manner known per se.

The gas mixture leaving the pre-dewatering column (7) passes via line (8) into a heat exchanger (9), where it is brought into countercurrent heat exchange with the return, which enters via line (16) and via line (10) into the pre-dewatering column (7) is returned. The gas mixture passes from the heat exchanger (9) via line (11) in a water-cooled condenser (12), in which it is cooled to about 35 ° C. The liquefied components pass via line (13) into containers (14), where they are collected. The portion of liquid exceeding a certain level in the collecting container (14) is pumped back into the pre-dewatering column (7) by means of pump (15) via line (16), heat exchanger (9) and line (10). After some time, the condensate accumulating in the sump (14) separates into two phases (17) and (18); From now on, the aqueous phase (17) is discharged via line (19) and only the organic phase (18) is wholly or partially via line (16), heat exchanger (9) and line (10) as reflux to the head of the pre-dewatering column (7) pumped back.

The gas mixture leaving the condenser (12) via line (20) is washed with the acetic acid introduced via line (51) in the scrubbing column (21) (circulating gas scrubber) and freed from uncondensed vinyl acetate fractions. The bottom outlet from the cycle gas scrubber (21) is separated, wherein a partial flow through the conduit (22) is conducted in Umpump and is returned with cooling by means of the heat exchanger (23) to the lower part of the cycle gas scrubber (21), while the other part of the Sump drain via the line (24) through a heat exchanger (25) is passed, in which the sump trigger to a temperature of at least 30 ° C, preferably from 60 to 120 ° C and in particular from 60 to 100 ° C, heated. The thus heated bottom draw is then pumped back to the lower part of the pre-dewatering column (7), preferably to the 2nd to 15th, in particular to the 5th to 10th floor, calculated from the bottom of the column.

The scrubbing column (21) via line (26) leaving residual gas or circulating gas (ethylene, unreacted oxygen and by-produced CO ₂ ), with the via line (35) supplied return gas, the predominantly ethylene and next CO ₂ , inert like Nitrogen and argon, and acetic acid and small amounts of vinyl acetate and ethyl acetate, combined, compressed by means of the cycle gas compressor (27) and returned via line (1) and the acetic acid evaporator (2) to the reactor (5). A portion of the recycle gas is removed for discharge of inert components via line (28) as exhaust gas. Fresh ethylene is fed via line (29) and fresh oxygen via line (30).

The obtained at the bottom of the pre-dewatering column (7) liquid, which consists mainly of vinyl acetate, acetic acid and water and contains almost all of the ethyl acetate is fed via line (31) a container (32), also referred to as Rohvinylacetatsammelbehälter, fed and relaxed, preferably a pressure of 0.02 to 0.2 MPa, in particular to a pressure of 0.1 to 0.15 MPa. The formed return gas, the predominantly ethylene and besides CO ₂ , inerts such as nitrogen and argon, as well as organic components such as acetic acid, is discharged via the line (33), combined with the recirculated from line (57) return gas, which has approximately the same composition, and after compression in the recycle gas compressor (34) via the line (35) with the recirculated gas (26) recycled from the cycle gas scrubber (21). The organic phase which accumulates in the crude vinyl acetate collecting tank (32) is withdrawn via line (36) and introduced into the second distillation column (37), also called the azeotrope column.

The overhead vapor of the second distillation column (37) is passed via line (38) in cooler (39) and condensed there. The condensate passing through the line (40) into the phase separator (41) separates into an aqueous phase (42), which is withdrawn via line (43) and an organic phase (44), which with the organic from the line (58) brought up Phase is merged. The combined in the phase separator (41) organic phase is removed by means of pump (45). A portion of the discharged organic phase is passed via the line (46) to the top of the Aezotropkolonne (37) and serves as reflux. The part not used as reflux is discharged via the line (47) and placed on a third distillation column (48), the dewatering column. The ethyl acetate passing via line (36) into column (37) is withdrawn from an enrichment zone above the bottom of column (37) via line (49). The bottom product of the column (37) contains almost completely obtained in the vinyl acetate working up acetic acid, at most 10 wt .-% water and small amounts of high boilers or polymers and only traces of vinyl acetate and ethyl acetate.

The aqueous acetic acid is withdrawn from the bottom of the column (37) via line (50) and split. Depending on the design of the wash column (21) and the temperature of the gas to be washed, different amounts of acetic acid are required as washing liquid. The one for the Acetic acid wash in step e) required fraction is fed via line (51) and pump (52) of the wash column (21). The remainder is fed via the pump (53) and line (3) to the acetic acid evaporator (2). Fresh acetic acid is added to the top of the acetic acid evaporator (2) via line (54) according to the amount of acetic acid consumed in the reaction, and at the same time serves as a washing solution for the recirculated acetic acid introduced via line (3), which is also referred to as back acetic acid.

The remainder of the organic phase (18) from collecting vessel (14) is fed via line (55), unless the entire organic phase (18) is used as reflux in the pre-dewatering column (7), to the flash tank (56). The return gas formed during the expansion to a pressure of 0.02 to 0.2 MPa, preferably to 0.1 to 0.15 MPa, is removed via line (57), combined with the return gas introduced via line (33) and after Compressed by the recycle gas compressor (34) via the line (35) fed back into the process.

The liquid obtained in the container (56) is passed via line (58) onto the phase separator (41), from where the combined organic phases partly as reflux via the line (46) to the azeotrope column (37) and partly as inflow via the line (47) to the third distillation column (48), also called dewatering column. The feed to the dewatering column is almost free of acetic acid.

The low boilers and last water residues contained in the overhead vapor of the column (48) are removed via line (59) and discharged from the work-up process.

The virtually anhydrous vinyl acetate obtained at the bottom of the column (48) is fed via line (60) to the fourth distillation column (61), also called the pure vinyl acetate column. The overhead vapor of this column goes over Line (62) to the capacitor (63). The resulting condensate is ethyl acetate free pure vinyl acetate. Via line (64) a very small portion of this vinyl acetate is returned as reflux into the column (61). Via line (65) pure vinyl acetate is removed. The bottom product of the column (61), which contains small amounts of ethyl acetate, polymers and high boilers, is recycled via line (66) and pump (67) to the column (37). From the acetic acid evaporator (2), in which ultimately all high-boiling components and polymers are recycled, a partial stream for polymer discharge is taken off via line (67).

The measure essential for the work-up process according to the invention consists in the return of the heated acetic acid bottoms effluent from the scrubbing column (21) to the lower part of the pre-dewatering column (7), wherein the already heated, acetic acid bottoms effluent from the scrubbing column is reheated. Surprisingly, this measure is associated with many advantages.

By virtue of this measure, the solubility of the gaseous components contained in the bottom outflow of the cycle gas scrubber, in particular of ethylene and carbon dioxide, which is expelled via the top of the first distillation column and is returned to the recycle gas at an early stage of the process, is reduced.

As a result, less return gas accumulates during the expansion, which is compressed up in the recirculating-gas compressor (34) with less expenditure of energy and is returned to the process. The return gas compressor is thus relieved.

By the task of the heated, acetic acid bottoms effluent from the wash column (21) to the lower part of the pre-dewatering column (7), a washing effect is achieved and almost all of the ethyl acetate is washed in the bottom of the pre-dewatering column (7) and on the Swamp run discharged. Only a very small proportion of ethyl acetate is entrained in the organic phase (18) which accumulates in the collection container (14). The stream withdrawn via line (55) therefore hardly contains any more ethyl acetate. Its proportion can therefore be increased in comparison to the known method of operation, whereby the load of the azeotrope column (37) is reduced, which also leads to further energy savings. Thus, the azeotrope can be operated at a significantly lower reflux ratio compared to the known procedure.

Also, a larger amount of water compared to the known method of operation is discharged via the head of the pre-dewatering column (7), whereby the amount of water in the container (14) can be increased. Therefore, the water removal via the pre-dewatering column (7) can be operated more effectively. Water is thus increasingly removed at an early stage of the process and later process stages for water removal are relieved.

Further, the total acetic acid is removed with the bottom of the azeotrope column (37), so that the feed to the dewatering column (48) and thus also to the pure vinyl acetate column (61) is acetic acid-free. Therefore, corrosion caused by acetic acid can be avoided in these parts of the system and less corrosion-resistant materials can be installed. By avoiding an acetic acid content in the feed to the pure vinyl acetate column (61) also reduces the distillation effort to obtain the pure vinyl acetate, since the separation of residual traces of acetic acid from vinyl acetate is very expensive. The distillation to obtain the pure vinyl acetate can therefore be operated with lower energy input and lower reflux ratio, which generally means a significant steam savings.

Claims (11)

1. A process for separating off vinyl acetate from a gas mixture formed in the reaction of ethylene with acetic acid and oxygen in the gas phase over catalysts comprising palladium or palladium compounds, comprising the following steps:

   a) introducing the gas mixture leaving the reaction zone into a first distillation column;

   b) cooling the gas mixture leaving the top of the first distillation column to from -20 to +50°C, with the condensate obtained separating into a water phase and an organic phase;

   c) taking off the water phase formed in step b);

   d) recirculating all or part of the organic phase formed in step b) as runback to the top of the first distillation column utilized in step a) and taking off part of the organic phase which is not used as runback;

   e) scrubbing the gas comprising vinyl acetate which is not condensed in step b) in a scrubbing column by means of at least 90% strength aqueous acetic acid and obtaining an acetic-acid solution comprising vinyl acetate at the bottom;

   f) feeding the bottom product comprising vinyl acetate, ethyl acetate, acetic acid and water from step a) into a collection vessel and depressurizing the pressurized liquid so as to form a gas;

   g) feeding the liquid obtained in the depressurization in step f) into a second distillation column and taking off a side stream comprising ethyl acetate from an enrichment zone above its bottom;

   h) utilizing all or part of the bottom product comprising acetic acid and water from step g) for the gas scrub in step e);

   i) cooling the overhead vapor from step g), with the condensate obtained separating into an aqueous phase and an organic phase;

   j) taking off the aqueous phase formed in step i);

   k) recirculating part of the organic phase formed in step i) as runback to the top of the second distillation column utilized in step g) and taking off the remainder; characterized in that

   l) part of the bottom product from the scrubbing column utilized in step e) is firstly cooled with pumped circulation and recirculated to the bottom of the scrubbing column utilized in step e), the remainder is taken off and the part taken off is heated to a temperature of at least 30 °C and fed into the lower part of the first distillation column utilized in step a);

   m) the remaining organic phase taken off in step d) is depressurized, the gas formed in the depressurization is combined with the gas formed in step f) and the combined gas is returned to the process;

   n) the organic phase obtained in step m) is combined with the organic phase obtained in step i) and the remaining part of the organic phase taken off in step k) which has not been used as runback is introduced into a third distillation column;

   o) the overhead product from the third distillation column in step n) is cooled and the low boilers obtained and the water obtained are separated off;

   p) the bottom product from the third distillation column in step n) is introduced into a fourth distillation column;

   q) pure vinyl acetate is taken off at the top of the fourth distillation column used in step p).
2. The process of claim 1, characterized in that the bottom product taken off from the scrubbing column is heated to a temperature of from 60°C to 120°C, preferably from 60°C to 100°C, in step l).
3. The process of claim 1 or 2, characterized in that the bottom product taken off in step l) is fed into the first distillation column utilized in step a) at the 2nd to 15th plate, preferably at the 5th to 10th plate, calculated from the bottom of the column.
4. The process of one or more of claims 1 to 3, characterized in that the recirculation of the bottom product taken off in step l) to the first distillation column utilized in step a) is carried out in such a way that the outflow from the bottom of the first distillation column has a temperature of from 80°C to 150°C.
5. The process of one or more of claims 1 to 4, characterized in that the bottom product from step a) is depressurized to a pressure of from 0.02 to 0.2 MPa, preferably from 0.1 to 0.15 MPa, in the collection vessel in step f).
6. The process of one or more of claims 1 to 5, characterized in that a residual organic phase taken off in step d) is fed to a depressurization vessel and depressurized to a pressure of from 0.02 to 0.2 MPa, preferably from 0.1 to 0.15 MPa, in step m).
7. The process of one or more of claims 1 to 6, characterized in that the cooling temperature in step b) and the proportion of the organic phase formed in step b) which is utilized as runback in step d) are selected so that virtually all the ethyl acetate is present in the bottom product from the first distillation column of step a).
8. The process of one or more of claims 1 to 7, characterized in that the part of the organic phase which is not used as runback in step d) is increased, with the organic phases combined in a phase separator in step n) containing as little ethyl acetate as possible.
9. The process of one or more of claims 1 to 8, characterized in that in step n) the amount of the combined organic phase which is recirculated as runback is only the amount required for the overhead vapor from the second distillation column to contain as little acetic acid and ethyl acetate as possible.
10. The process of one or more of claims 1 to 9, characterized in that the amount of the cooled overhead product returned as runback to the third distillation column in step o) is the amount necessary for a sufficient part of the low boilers and water to be separated off.
11. The process of one or more of claims 1 to 10, characterized in that the gas mixture leaving the reaction zone is firstly cooled to from 115°C to 150°C by means of the colder circulating gas in the countercurrent heat exchanger and only then introduced into the first distillation column in step a).

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